feat(printer): add write and display formatting

Sources/KiteCore/Printer.swift

@@ -0,0 +1,92 @@
+public func displayString(_ value: Value) -> String {
+    render(value, write: false)
+}
+
+public func writeString(_ value: Value) -> String {
+    render(value, write: true)
+}
+
+func formatReal(_ d: Double) -> String {
+    if d.isNaN { return "+nan.0" }
+    if d.isInfinite { return d > 0 ? "+inf.0" : "-inf.0" }
+    if d == d.rounded() && abs(d) < 1e15 {
+        return String(format: "%.1f", d)
+    }
+    return String(d)
+}
+
+private func render(_ value: Value, write: Bool) -> String {
+    switch value {
+    case .null:
+        return "()"
+    case .bool(let b):
+        return b ? "#t" : "#f"
+    case .integer(let n):
+        return String(n)
+    case .rational(let n, let d):
+        return "\(n)/\(d)"
+    case .real(let d):
+        return formatReal(d)
+    case .char(let c):
+        if write { return writeChar(c) }
+        return String(c)
+    case .string(let s):
+        if write { return "\"" + escapeString(s) + "\"" }
+        return s
+    case .symbol(let s):
+        return baseName(s)
+    case .pair:
+        return renderList(value, write: write)
+    case .builtin(let b):
+        return "#<procedure:\(b.name)>"
+    case .closure(let l):
+        return "#<procedure:\(l.name)>"
+    case .continuation:
+        return "#<continuation>"
+    case .syntax:
+        return "#<syntax>"
+    case .unspecified:
+        return ""
+    case .eof:
+        return "#<eof>"
+    }
+}
+
+private func renderList(_ value: Value, write: Bool) -> String {
+    var parts: [String] = []
+    var cur = value
+    while case .pair(let p) = cur {
+        parts.append(render(p.car, write: write))
+        cur = p.cdr
+    }
+    if case .null = cur {
+        return "(" + parts.joined(separator: " ") + ")"
+    }
+    return "(" + parts.joined(separator: " ") + " . " + render(cur, write: write) + ")"
+}
+
+private func writeChar(_ c: Character) -> String {
+    switch c {
+    case " ": return "#\\space"
+    case "\n": return "#\\newline"
+    case "\t": return "#\\tab"
+    case "\r": return "#\\return"
+    case "\u{0}": return "#\\null"
+    default: return "#\\\(c)"
+    }
+}
+
+private func escapeString(_ s: String) -> String {
+    var out = ""
+    for c in s {
+        switch c {
+        case "\"": out += "\\\""
+        case "\\": out += "\\\\"
+        case "\n": out += "\\n"
+        case "\t": out += "\\t"
+        case "\r": out += "\\r"
+        default: out.append(c)
+        }
+    }
+    return out
+}